Antiwear agents and viscosity index improvers are known to be added to lubricating oil compositions to improve the wear performance and viscosity index of the lubricant respectively.
Common antiwear additives include ash-forming phosphorus compounds and/or ashless forming compounds. Common viscosity index improvers include polymers of methacrylates, acrylates, olefins, or maleic-anhydride styrene copolymers and esterified derivatives thereof. The viscosity index improvers tend to incorporate ester functional groups in pendant/grafted/branched groups. The ester functional groups may be derived from linear alkyl alcohols with 1 to 40 carbon atoms. Recent attempts have been made to produce viscosity index improvers where the ester functional groups have a degree of branching. However, such viscosity index improvers have poor shear stability, viscosity index control and low temperature viscosity.
U.S. Pat. No. 6,746,993 discloses a viscosity index improver defined as a polymer with a solubility parameter of 8.6-9.4, a crystallisation temperature of −15° C., or less and a steric hindrance factor of 0 to 13. The polymer comprises alkyl alkenyl ethers and C1-40 alkyl methacrylates, of which some may be β-branched. The viscosity index improver is suitable for gear oils, hydraulic fluids, automatic transmissions and engine oils.
U.S. Pat. No. 5,763,374 discloses lubricating oil compositions containing a copolymer composed of 20-70% of alkyl acrylates, 30-80% alkyl methacrylates. The lubricating oil may be a gear oil or an engine lubricant.
U.S. patent application 2004/0077509 discloses a viscosity index improver polymer suitable for gear oils, transmissions, traction oils, hydraulic oil and engine oils. Further the polymer provides an improved shear stability and low temperature viscosity. The polymer is composed of (meth)acrylates derived from branched alcohols. The branched ester groups contain C18-36 alkyl groups, with the proviso that the group does not contain a methylene group containing more than 16 carbon atoms. The polymer further contains 5-90% of either a C8-17 alkyl (meth)acrylate or C18-24 alkyl (meth)acrylate; and 5-50% of a hydroxy, or amide or carboxyl containing monomer. The monomer with branched ester groups may be present at 5 to 90%, or 10 to 70% or 20 to 60%. Further disclosed are lubricating compositions containing said polymer and a zinc or molybdenum antiwear agent.
Given the performance of the viscosity index improvers of the prior art, it would be desirable to have lubricating compositions capable of providing acceptable/improved wear performance and at least one of acceptable/improved shear stability, viscosity index control and low temperature viscosity. The present invention provides such a lubricating composition.